An alternative approach to the design of nonhydrostatic numerical weather prediction (NWP) models is presented. Instead of extending mesoscale nonhydrostatic modeling concepts to the synoptic scales and beyond, a hydrostatic NWP model using the mass-based σ vertical coordinate has been extended to include the nonhydrostatic motions, preserving the favorable features of the hydrostatic formulation. In order to do so, the system of nonhydrostatic equations was split into two parts: (a) the part that corresponds to the hydrostatic system, except for higher-order corrections due to the vertical acceleration, and (b) the system of equations that allows computation of the corrections appearing in the first system due to the vertical acceleration. This procedure does not require any linearization or approximation. With this approach, the nonhydrostatic dynamics has been introduced through an add-on nonhydrostatic module. The separation of the nonhydrostatic contributions shows in a transparent way where... Abstract An alternative approach to the design of nonhydrostatic numerical weather prediction (NWP) models is presented. Instead of extending mesoscale nonhydrostatic modeling concepts to the synoptic scales and beyond, a hydrostatic NWP model using the mass-based σ vertical coordinate has been extended to include the nonhydrostatic motions, preserving the favorable features of the hydrostatic formulation. In order to do so, the system of nonhydrostatic equations was split into two parts: (a) the part that corresponds to the hydrostatic system, except for higher-order corrections due to the vertical acceleration, and (b) the system of equations that allows computation of the corrections appearing in the first system due to the vertical acceleration. This procedure does not require any linearization or approximation. With this approach, the nonhydrostatic dynamics has been introduced through an add-on nonhydrostatic module. The separation of the nonhydrostatic contributions shows in a transparent way where...